1. Field of the Invention
The present invention generally relates to digital voice communications systems and, more particularly, to a line spectral frequency vector quantizer for code excited linear predictive (CELP) speech encoders. Such devices are commonly referred to as "codecs" for coder/decoder. The invention has particular application in air-to-ground telephony but may be advantageously used in any product line that requires speech compression for communications.
2. Description of the Prior Art
Cellular telecommunications systems in North America are evolving from their current analog frequency modulated (FM) form towards digital systems. Digital systems must encode speech for transmission and then, at the receiver, synthesize speech from the received encoded transmission. For the system to be commercially acceptable, the synthesized speech must not only be intelligible, it should be as close to the original speech as possible.
Codebook Excited Linear Prediction (CELP) is a technique for low rate speech coding. The basic technique consists of searching a codebook of randomly distributed excitation vectors for that vector which produces an output sequence (when filtered through pitch and linear predictive coding CLPC) short-term synthesis filters) that is closest to the input sequence. To accomplish this task, all of the candidate excitation vectors in the codebook must be filtered with both the pitch and LPC synthesis filters to produce a candidate output sequence that can then be compared to the input sequence. This makes CELP a very computationally-intensive algorithm, with typical codebooks consisting of 1024 entries, each 40 samples long. In addition, a perceptual error weighting filter is usually employed, which adds to the computational load.
Fast digital signal processors (DSPs) have helped to implement very complex algorithms, such as CELP, in real-time. A number of techniques have been considered to mitigate the computational load of CELP encoders. For example, one strategy is a variation of the CELP algorithm called Vector-Sum Excited Linear Predictive Coding (VSELP). One VSELP codebook search method is disclosed in U.S. Pat. No. 4,817,157 by Gerson. Gerson addresses the problem of extremely high computational complexity for exhaustive codebook searching. The Gerson technique is based on the recursive updating of the VSELP criterion function using a Gray code ordered set of vector sum code vectors. The optimal code vector is obtained by exhaustively searching through the Gray code ordered code vector set. The Electronic Industries Association (EIA) in August 1991 adopted the Gerson VSELP codebook search method for the dual-mode mobile station, base station cellular telephone system compatibility standard. Although the Gerson search technique provides a notable reduction in computational complexity, it still requires a relatively expensive digital signal processor to implement, making the cost of the transceiver high.
Although the VSELP codebook search method has been adopted as the standard for mobile cellular telephone systems in the United States, no such standard presently exists for air-to-ground telephony. As the technology for this application of digital communications evolves, it is desirable to develop improved CELP processing techniques that would result in the best possible service for a competitive cost.